1. Field of the Invention
The present invention relates to a grease composition having excellent heat resistance, load carrying capacity, water resistance and lubricating life, and a grease composition having excellent heat resistance, load carrying capacity, water resistance, lubricating life and biodegradability. Particularly, the present invention relates to a grease composition, which can preferably be used for lubrication of the rolling and abrading portions of iron manufacturing machines, construction machines, cars and others, which are used under high load. Moreover, the present invention relates to a rolling apparatus which is long-lived although it is used under strict conditions, and a rolling apparatus which is long-lived although it is used under strict conditions and produces little adverse effect to the environment.
Furthermore, the present invention relates to a grease composition having excellent heat resistance, oxidation stability and lubricating life, and a grease composition having excellent heat resistance, oxidation stability, lubricating life and biodegradability. Further, the present invention relates to a rolling apparatus which is long-lived, and a rolling apparatus which is long-lived and produces little adverse effect to the environment.
Still further, the present invention relates to a grease composition, which generates a few dusts at a high temperature and has excellent high-temperature performance. Further, the present invention relates to a rolling apparatus, which has low torque and excellent acoustic performance and generates a few dusts at a high temperature, and in particular, the present invention relates to a rolling apparatus which is preferably used in electronic intelligence equipment, semiconductor manufacturing machines and others.
Still further, the present invention relates to a grease composition, which has excellent performance under a high-speed and high-temperature environment and can significantly improve the seizuring life of a rolling apparatus. Further, the present invention relates to a rolling apparatus having excellent seizuring life under a high-speed and high-temperature environment, and it particularly relates to a rolling apparatus which is preferably used in cars, rolling stocks, electric motors, iron manufacturing machines, machine tools and others.
2. Description of the Related Art
(1) With the development of mechanical technology in recent years, mechanical devices tend to be directed toward downsizing, weight saving and high-speed rotation. For this reason, mechanical portions such as a bearing and a gear become increasingly exposed at a high temperature.
On the other hand, with recent requirements for resources and power saving efforts, maintenance-free mechanical devices have been developed. For this reason, greases used in such mechanical devices are required to have heat resistance and lubricating life, which are more excellent than those of the conventional mechanical devices. Of these mechanical devices, iron manufacturing machines (e.g. a work roll of a rolling mill, etc.), construction machines, cars (e.g. a wheel bearing, etc.) and others are often used in contact with water, and so greases used in these machines are required to have excellent water resistance, load carrying capacity and durability.
At present, examples of a grease for high-temperature use include a complex metallic soap grease comprising, as a thickener, a complex metallic soap such as Ca, Al and Li complexes. Moreover, examples of a grease for high-temperature use also include an organic grease comprising an organic compound such as polyurea, a terephthalamic acid metal salt or polytetrafluoroethylene (PTFE), an inorganic grease comprising an inorganic compound such as bentonite or silica gel, and others.
Furthermore, as a general-purpose grease, a grease comprising a metallic soap, especially a lithium soap as a thickener has broadly been used. This is because the metallic soap has excellent thickening property and operability as well as being low-cost, and also because the lithium soap has well-balanced properties such as heat resistance, torque property, acoustic property and structural stability, which are required as properties for a grease.
Still further, as a grease having excellent water resistance, rust protection, heat resistance and load carrying capacity, there is known a calcium sulfonate complex grease, which comprises, as a thickener (an organic thickener), a calcium sulfonate complex containing calcium carbonate (Japanese Patent Publication No. 5-8760).
However, each of these greases has a problem. For example, once the above described complex metallic soap grease contains a large amount of water, then it cannot maintain the grease structure for a long time.
Moreover, while the metallic soap grease has excellent lubricating property, it cannot maintain the grease structure for a long time at a high temperature. Although the lithium soap grease is the most heat resistant, it has significantly decreased lubricating life at a high temperature over 130° C.
Furthermore, the organic grease comprising polyurea is likely to cause hardening phenomenon, and the organic grease comprising a terephthalamic acid metal salt causes large oil separation though it has excellent heat resistance. Further, the inorganic grease comprising bentonite or silica gel has insufficient lubricating and anticorrosive properties.
Still further, the calcium sulfonate complex grease comprises calcium carbonate, which is dispersed in a colloid state therein, and it has a small particle size of 0.2 to 0.5 μm. Accordingly, this grease has a low ability to maintain oil. When this grease is used for a long time for lubrication of rolling and abrading portions such as a bearing or gear, which are used under particularly high load, this grease causes problems regarding load carrying capacity such that uneven wear occurs in the above portions or that the temperature abnormally increases, and further, this grease has insufficient durability. Still further, since this grease tends to harden when water is mixed therein, in some cases, this grease has a problem with durability or pressure transmitting property.
On the other hand, when each of the above greases is used in various types of mechanical devices, the grease is often released to the environment, and thus the grease causes a risk of having adverse effect on the environment. To solve this problem, Japanese Patent Laid-Open Nos. 5-86389, 6-1989 and 8-20789 propose greases in which a polyol ester oil or plant oil is used as a base oil so as to impart biodegradability to the greases. However, the greases described in the above publications have a problem that their lubricating life and oxidation stability are insufficient.
Thus, it is a first object of the present invention to solve the above described problems of the prior art techniques and to provide a grease composition having excellent heat resistance, load carrying capacity, water resistance and lubricating life, and a grease composition having excellent heat resistance, load carrying capacity, water resistance, lubricating life and biodegradability.
Moreover, it is also the first object of the present invention to provide a rolling apparatus which is long-lived although being used under strict conditions, and a rolling apparatus which is long-lived although being used under strict conditions and has little adverse effect on the environment.
It is a second object of the present invention to solve the above described problems of the prior art techniques and to provide a grease composition having excellent heat resistance, oxidation stability and lubricating life, and a grease composition having excellent heat resistance, oxidation stability, lubricating life and biodegradability.
Moreover, it is also the second object of the present invention to provide a rolling apparatus which is long-lived, and a rolling apparatus which is long-lived and has little adverse effect on the environment.
(2) Generally, rolling apparatuses (e.g. rolling bearings, linear guide apparatuses, ball screws, etc.), which are used in electronic intelligence equipment, semiconductor manufacturing machines and others such as a hard disk drive (hereinafter referred to as HDD) or laser beam printer (LBP), are required to have various performances such that these apparatuses generate a few dusts, have low torque and excellent acoustic performance, and are long-lived.
Moreover, with the development of high-precision electronic intelligence equipment and semiconductors in recent years, the use conditions (temperature, speed, etc.) of rolling apparatuses, which are used in electronic intelligence equipment and semiconductor manufacturing machines, have become increasingly strict. Under such strict use conditions, the above stated various types of performance are required to be excellent.
For example, in the case of electronic intelligence equipment such as HDD used under a clean atmosphere, if the fine particles of a gaseous oil or grease are released from the inside of a bearing when a rolling apparatus rotates, the fine particles pollute a recording medium or the like and cause mechanical errors. Therefore, it is the most important to suppress the amount of dusts generated.
As a grease composition enclosed in such a rolling bearing for HDD, Undock C (Trade name) produced by ExxonMobil, which is a grease comprising a mineral oil as a base oil and a sodium complex soap as a thickener, is well known, and this product has been used over 20 years because it causes a small amount of dusts.
Nevertheless, in the above described sodium complex soap-mineral oil grease, the dispersibility of the thickener is insufficient and so it hardly becomes homogenous, and thereby problems regarding acoustic and vibration performances occur in the early stage of rotation of the rolling bearing. In addition, since this grease has high hygroscopicity and gets hard over time, and thereby the flowability in the rolling bearing decreases, this grease also has the problem of causing insufficient lubrication and readily generating abnormal sounds generated from the cage.
Therefore, when special emphasis is placed on low torque and low noise, a grease comprising a lithium soap as a thickener and an ester oil as a base oil is used at times. However, since this grease is easily dispersed (causes a large amount of dusts), there is a great risk that the grease might pollute a recording medium or the like when it is used in a rolling bearing for HDD. Moreover, since the temperature used for the rolling bearing for HDD is more and more increased, the above described lithium soap-esteroil grease cannot be applied in some cases.
As a grease having excellent high-temperature performance, a urea grease comprising a urea compound as a thickener is known. However, since this urea grease generally does not have sufficient acoustic performance, when emphasis is placed on acoustic performance, it is difficult to use this grease. Under these circumstances, urea greases having improved acoustic performance have been proposed (Japanese Patent Publication No. 6-92592 and Japanese Patent Gazette No. 2864473), but these greases also have a problem that they are more expensive than greases comprising a metallic soap as a thickener.
Thus, it is a third object of the present invention to solve the above described problems of the prior art techniques and to provide a grease composition which causes a few dusts even at a high temperature and has excellent high-temperature performance. It is also the third object of the present invention to provide a rolling apparatus which has low torque and excellent acoustic performance and generates a few dusts even at a high temperature.
(3) In recent years, various types of machines such as cars, rolling stocks, electric motors, iron manufacturing machines and machine tools tend to be directed toward downsizing, weight saving and speeding up. With this phenomenon, a grease for a rolling apparatus (e.g. a rolling bearing, a linear guide apparatus, a ball screw, etc.), which is incorporated into each of the above various machines, is used at a higher speed and a higher temperature, and the use conditions of this grease become extremely severe.
For example, in a car equipped with a disk brake, a grease used in the wheel bearing could have a high temperature of 150 to 200° C. due to friction heat generated as a result of actuation of the brake. If the grease has such a high temperature, there is a risk that the grease might soften, flow and run out of a hub, or deterioration by oxidation might quickly progress, resulting in insufficient lubrication.
The electric motor of a rolling stock has been shifted from a direct-current motor to an induction motor, and therefore the use conditions of a grease for the electric motor are directed to the use under a high-speed and high-temperature environment. That is to say, the dmN value of a bearing used in an electric motor increases from 300,000 to 800,000, and with this change, the temperature of the bearing also increases from about 80° C. to about 100° C. It should be noted that, herein, dm of the dmN value means the pitch circle diameter (unit: mm) of a rolling bearing, and N of the dmN value means rotation speed (unit: min−1).
Under these circumstances, a grease used in a rolling apparatus incorporated in each of the above various types of machines is required to have excellent performance even extremely severe conditions such as a high speed and a high temperature.
Moreover, for each of the above various types of machines, there are not only requirements for downsizing, weight saving and speeding up, but there are also strong requirements for maintenance-free. Accordingly, a rolling apparatus incorporated in each of the above various types of machines is required to have excellent seizuring life.
As a grease composition used in the rolling apparatus incorporated in each of the above various types of machines, a grease composition comprising a metallic soap such as a lithium soap has previously been used, but in recent years, a grease composition comprising a complex metallic soap has largely been used to improve heat resistance.
An example of such a grease composition comprising a complex metallic soap includes a grease composition comprising a lithium complex soap, which is described in Japanese Patent Gazette Nos. 2544952 and 2862612, and Japanese Patent Laid-Open No. 2001-187891.
However, the above described grease composition comprising a lithium complex soap has a problem that the grease composition hardens over time. Similarly, a grease composition comprising a calcium complex soap has the same problem. A grease composition comprising an aluminum complex soap has a problem regarding long-term stability. Thus, grease compositions comprising the conventional complex metallic soaps have various problems.
As a grease composition used under a high-speed and high-temperature environment, a grease composition comprising urea as a thickener has previously been known. This urea grease has been used in a bearing, which is incorporated in the electrical component or engine accessory equipment of a car. However, this urea grease has a problem that, when the grease is used in a ball bearing, the grease can impart a sufficient seizuring life to the ball bearing, but when the grease is used in a roller bearing, in some cases the grease cannot impart a sufficient seizuring life to the roller bearing.
Thus, it is a fourth object of the present invention to solve the above described problems of the prior art techniques, and to provide a grease composition which has excellent performance even under a high-speed and high-temperature environment and can impart an excellent seizuring life to a rolling apparatus. Moreover, it is also the fourth object of the present invention to provide a rolling apparatus having excellent seizuring life even under a high-speed and high-temperature environment.
(4) As stated above, greases are classified into the following 4 types according to the types of a thickener:
(i) a metallic soap grease comprising a metallic soap such as Li, Ca, Na or Al;
(ii) a complex metallic soap grease comprising a complex metallic soap such as Li, Ca, Na and Al;
(iii) an organic grease comprising an organic compound (an organic thickener) such as polyurea, a terephthalamic acid metal salt or a calcium sulfonate complex; and
(iv) an inorganic grease comprising an inorganic compound (an inorganic thickener) such as bentonite or silica gel.
However, as described in the “Lubrication Control Manual Book” edited by the Japan Lubricating Oil Society and the Lubrication Control Diffusion Task Force (the Japanese Lubricating Oil Society, published Mar. 20, 1990) and “A Brief History of Lubricating Greases” written by Arthur T. Polishuk (Llewellyn& McKane, Inc., published 1998), it has previously been thought that, when different types of greases are mixed, the properties of the greases are altered and deteriorated in many cases. This deterioration naturally occurs when 2 types from among the above 4 types of greases are mixed, but it could also occur when 2 types of metallic soap greases are mixed, in which the type of metals is different.
When a significant deterioration occurs in a grease, the grease structure is destroyed, significant softening of the grease or decrease of the dropping point occurs, and the grease is liquefied at times. Moreover, there are also cases where additives which are added to a grease, act on one another and reduce the performance of the grease. For these reasons, it has generally been believed that the mixing of greases should be prevented as much as possible.
Greases used in rolling apparatuses such as a rolling bearing, a ball screw, a linear guide apparatus and a linear bearing are required to have various performances such as lubricating performance, load carrying capacity, heat resistance, water resistance, low torque and few dusts generating property. To obtain such a grease having many excellent performances, a method for mixing greases each having one of the above performances is considered to be effective. However, probably because it has generally been believed that the mixing of greases comprising different types of thickeners is not good, the above method has seldom been used. Only a few examples are found, such as a combined use of an N-substituted terephthalamic acid metal salt and polyurea disclosed in Japanese Patent Publication No. 6-31375 and a combined use of a sodium soap and a lithium soap disclosed in Japanese Patent Laid-Open No. 2001-3074.
Although two types of thickeners are used in combination, if the excellent performances of both thickeners consist with each other, the above described grease having many excellent performances can be obtained. Thus, the present invention adopts a method involving the combined use of two different types of thickeners to achieve the above described first to fourth objects.